[bug report] blk-mq-tag: cleanup the normal/reserved tag allocation

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Jens Axboe,

This is a semi-automatic email about new static checker warnings.

The patch 4941115bef2b: "blk-mq-tag: cleanup the normal/reserved tag 
allocation" from Jan 13, 2017, leads to the following Smatch 
complaint:

block/blk-mq-tag.c:142 blk_mq_get_tag()
	 warn: variable dereferenced before check 'data->hctx' (see line 102)

block/blk-mq-tag.c
   101	{
   102		struct blk_mq_tags *tags = blk_mq_tags_from_data(data);
                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
We added a new "data->hctx" dereference here.

   103		struct sbitmap_queue *bt;
   104		struct sbq_wait_state *ws;
   105		DEFINE_WAIT(wait);
   106		unsigned int tag_offset;
   107		int tag;
   108	
   109		if (data->flags & BLK_MQ_REQ_RESERVED) {
   110			if (unlikely(!tags->nr_reserved_tags)) {
   111				WARN_ON_ONCE(1);
   112				return BLK_MQ_TAG_FAIL;
   113			}
   114			bt = &tags->breserved_tags;
   115			tag_offset = 0;
   116		} else {
   117			bt = &tags->bitmap_tags;
   118			tag_offset = tags->nr_reserved_tags;
   119		}
   120	
   121		tag = __blk_mq_get_tag(data->hctx, bt);
   122		if (tag != -1)
   123			goto found_tag;
   124	
   125		if (data->flags & BLK_MQ_REQ_NOWAIT)
   126			return BLK_MQ_TAG_FAIL;
   127	
   128		ws = bt_wait_ptr(bt, data->hctx);
   129		do {
   130			prepare_to_wait(&ws->wait, &wait, TASK_UNINTERRUPTIBLE);
   131	
   132			tag = __blk_mq_get_tag(data->hctx, bt);
   133			if (tag != -1)
   134				break;
   135	
   136			/*
   137			 * We're out of tags on this hardware queue, kick any
   138			 * pending IO submits before going to sleep waiting for
   139			 * some to complete. Note that hctx can be NULL here for
   140			 * reserved tag allocation.
   141			 */
   142			if (data->hctx)

We're in a loop here so it's possible we don't need to check the first
item in the list, but it's kind of new so I thought I would forward the
message anyway even though it might be a false positive.

   143				blk_mq_run_hw_queue(data->hctx, false);
   144	

regards,
dan carpenter